Does sleeping too much or too little make you stupid? JAMA sub-journal study

Sleeping is a science! Recently, scholars from Peking University First Hospital and Peking University Clinical Research Institute published a new study in JAMA sub-journal, showing that sleeping too little or too much may lead to cognitive decline.

In the study, insufficient sleep was defined as sleeping ≤4 hours per night, while excessive sleep was considered sleeping ≥10 hours per night.

The researchers suggest that people who are sleep deprived or sleep too much should have their cognitive function monitored.

According to the National Sleep Foundation, sleep is essential because it allows your body and mind to recharge. Proper sleep also helps you stay healthy and prevent disease. Without enough sleep, the brain cannot function properly, affecting concentration, clear thinking, and memory processing.

However, the mechanisms behind these links remain unclear. The authors note that too much sleep may be linked to inflammation. At the same time, too little sleep may increase levels of amyloid plaques and tau proteins in cerebrospinal fluid, which are hallmarks of Alzheimer's disease.

The researchers also noted that during sleep, the brain's glymphatic system is more active than at any other time in the circadian cycle, effectively clearing excess toxins, including amyloid beta peptide. Each person may have some optimal balance between sleep and amyloid clearance, where too much or too little sleep can lead to an imbalance.

Optimizing sleep and amyloid clearance may well be as much of a treatable contributor to cognitive decline in old age as sleep apnea.

The study included 20,056 people, including 9,254 participants from the English Longitudinal Study of Ageing and 10,811 from the China Health and Retirement Longitudinal Study.

The researchers found that in a follow-up of 100,000 people-years, cognitive scores declined faster in those who slept ≤4 hours per night and ≥10 hours per night, that is, there was a U-shaped relationship between sleep duration and cognitive scores.

The editorial also pointed out that more and more studies have found a U-shaped relationship between sleep duration and cognition, with both short and long sleep duration being associated with poor cognitive ability. However, the meaning of this U-shaped relationship is still unclear, partly due to limitations in research design.

The editorial points out that to truly determine the impact of sleep on cognition, it is necessary to consider both sleep quality and sleep quantity in addition to studying sleep duration.

Nearly 20 years have passed since sleep duration was first suggested to be associated with cognitive health in older adults, and better study designs and more robust measures are needed to clarify the relationship.

Source: [1]Ma Y, Liang L, Zheng F, Shi L, Zhong B,Xie W. Association Between Sleep Duration and Cognitive Decline. JAMA NetwOpen. 2020, 3(9):e2013573.

[2]Leng Y, Yaffe K. Sleep Duration andCognitive Aging-Beyond a U-Shaped Association. JAMA Netw Open. 2020 Sep1;3(9):e2014008.

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